I've got the same TH and have the same problem especially if the sun directly hits the garage area. So far I have replaced the bathroom vent with one of these- which allowed us to control the bathroom temp but really didnt help with the garage. I've tried to shut the air dumps but that doesnt seem to any increase the flow on the other vents in the main area or the garage.
When I pulled the covers off the bedroom AC last night I found the divider between the supply and return wasn't positioned correctly. I used some of the foil tape to hold it properly, and re-taped the joints into main duct. I re-taped all the vents I could reach w/o a step ladders so far. I'm thinking that the main area AC is likely the same and looking to tackle that this weekend.
"No time to do it right, always time to do it twice...."